2P: Bonus Light of Dawn triggers Dawn's Light, instantly healing your current friendly target within 0 to 40 yds for [ 1 + 300% of Spell Power ].4P: Light of Dawn triggers up to 40% reduced cooldown based on the current health of the target of your Dawn's Light. More reduced cooldown when used on a lower health target.

So yeah, if you are a healer you might know why this might be a problem.Personally I find two big reasons:

1). Almost every healer uses either mouseover macros, clique or a similar playstyle when healing because it can be more efficient and you waste less time not targeting players every time you heal. This can be worked around with T20 with some macros like /target @mouseover, targetlasttarget etc in your LoD macro. I don't get a good feeling about having to do that though.

2). On top of that and an bigger issue, Light of Dawn is a spell you want to aim towards a group of people in the raid, as it's a cone-heal, and aiming/moving your character is usually done, not by keyboardturning, but by moving your mouse off the raidframes and rightclicking to turn your character and shooting off your Light of Dawn. Adding targeting someone before doing this and hoping they don't get healed before you can get off your LoD (since you can't target them and instantly LoD when needing to aim the heal).

tldr, 2P makes the set awkward to use as you need to rotate your character to get good hit with Light of Dawn at the same time as you need to target a friendly player.